This invention relates to a rotation angle sensor which uses permanent magnets which output a physical rotation angle as an electrical signal.
In the prior art, this type of rotation angle sensor comprises permanent magnets 4, 5 which rotate together about a rotation centreline (centerline) A, and a flux density detecting unit 10 which remains stationary relative to these magnets as shown in FIG. 8. In the permanent magnets 4, 5, unlike poles are arranged opposite each other, and are disposed symmetrically with respect to the rotation centreline A. The flux density detecting unit 10 is interposed between the permanent magnets 4, 5.
The flux density detecting unit 10 may for example be a hall (Hall) element which outputs a voltage according to the flux density which varies according to the relative rotation of the permanent magnets 4, 5.
In this type of prior art rotation angle sensor, the cross-sectional shape of the permanent magnets 4, 5 is square and opposite faces thereof are parallel, so the flux density distributed between the permanent magnets 4, 5 increases towards a magnet centreline B, as shown in FIG. 5. The output variation of the flux density detecting unit 10 depends not only on the variation amount of the flux input angle, but also on the relative positions of the permanent magnets 4, 5, i.e., on the offset amount from the magnet centreline B. Therefore, the output characteristics of the flux density detecting unit 10 are not linear but nonlinear relative to the displacement, as shown in FIG. 9, and this leads to a detection error.
It is therefore an object of this invention to provide a rotation angle sensor having high precision output performance without increasing cost.
The rotation angle sensor of this invention comprises a pair of permanent magnets with their opposite poles facing each other, and a magnetic flux density detecting unit which performs relative rotation between the permanent magnets. The opposite magnetic pole surfaces of the pair of permanent magnets are formed in a curved shape.
In this invention, it is preferable that the opposite magnetic pole surfaces of the permanent magnets are hollowed out to form concave depressions. Alternatively, the opposite magnetic pole surfaces of the permanent magnets may bulge outwards to form convex surfaces.
In this invention, a pair of hall elements may be disposed on either side of the rotation centreline as the flux density detecting unit.
According to this invention, by forming the opposite magnetic pole surfaces of the permanent magnets in a curved shape, the flux density between the permanent magnets may be distributed in any way desired, there is no need to use costly materials for the permanent magnets, and the required high detection precision can be obtained.
According to this invention, the flux density between the permanent magnets can be made uniform and the sensor output resolution can be made constant without using costly materials for the permanent magnets.
According to this invention, the flux density between the permanent magnets can be concentrated in the vicinity of the magnet centreline, a high resolution area in which the sensor output varies sharply relative to displacement of the flux density detecting unit can be set, and the detection precision of the rotation angle sensor can be increased.
Further, according to this invention, the hall elements are not disposed on the rotation centreline, so a suitable distribution of flux density can be obtained and the output voltage from the hall elements can be extracted without error.